Dynamic surface effect on boiling of aqueous surfactant solutions

Abstract

Boiling of aqueous surfactant solutions is known to be drastically different from that of pure water. The experimental results showed that a small amount of surfactant additive makes the nucleate boiling heat transfer coefficient, h b, considerably higher, and that there is an optimum additive concentration for higher heat fluxes. Beyond this optimum point, further increase in additive concentration makes h b lower. In this work, the effect of surfactant additive on boiling is attributed to the departure from equilibrium surface tension which is produced by extension of the vapor-liquid interface during growth and coalescence of vapor bubbles in the vicinity of boiling surface. The surface tension of a surfactant solution is higher than the static value because the surfactant component cannot diffuse to the adsorbed layer promptly. This dynamic surface effect can be expressed by the Y (≡c(dσ/dc) 2) value which is shown to be a representation of the elasticity of an adsorbed film as well. The similarity between the increment of h b due to the addition of surfactant and the Y curves indicates that the dynamic surface effect may play an important role in the boiling process of aqueous surfactant solutions.